Early work relating to Wireless Location Systems is described in U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. Further enhancements of the system disclosed in the '144 patent are disclosed in U.S. Pat. No. 5,608,410, Mar. 4, 1997, “System for Locating a Source of Bursty Transmissions.” Both of these patents are assigned to TruePosition, Inc., the assignee of the present invention. TruePosition has continued to develop significant enhancements to the original inventive concepts.
Over the past few years, the cellular industry has increased the number of air interface protocols available for use by wireless telephones, increased the number of frequency bands in which wireless or mobile telephones may operate, and expanded the number of terms that refer or relate to mobile telephones to include “personal communications services”, “wireless”, and others. The air interface protocols now used in the wireless industry include AMPS, N-AMPS, TDMA, CDMA, GSM, TACS, ESMR, GPRS, EDGE, UMTS WCDMA, and others.
The viability and value of Wireless Location System technology has been extensively demonstrated. In 2004 and 2005, TruePosition (the assignee of the present invention) installed E-911 Wireless Location Systems in more than 40,000 Base Transceiver Stations (BTS), providing emergency location coverage for wireless subscribers across the continental United States.
The wireless communications industry has acknowledged the value and importance of the Wireless Location System. In June 1996, the Federal Communications Commission issued requirements for the wireless communications industry to deploy location systems for use in locating wireless 9-1-1 callers. Widespread deployment of these systems can reduce emergency response time, save lives, and save enormous costs because of reduced use of emergency response resources. In addition, surveys and studies have concluded that various wireless applications, such as location sensitive billing, fleet management, and others, will have great commercial value in the coming years.
As mentioned, the wireless communications industry uses numerous air interface protocols in different frequency bands, both in the U.S. and internationally. In general, neither the air interface nor the frequency bands impact the Wireless Location System's effectiveness at locating wireless telephones.
All air interface protocols use two categories of channels, where a channel is defined as one of multiple transmission paths within a single link between points in a wireless network. A channel may be defined by frequency, by bandwidth, by synchronized time slots, by encoding, shift keying, modulation scheme, or by any combination of these parameters.
The first category, called control or access channel, is used to convey information about the wireless telephone or transmitter, for initiating or terminating calls, or for transferring bursty data. For example, some types of short messaging services transfer data over the control channel. Different air interfaces use different terminology to describe control channels but the function of the control channels in each air interface is similar.
The second category of channel, known as voice or traffic channel, typically conveys voice or data communications over the air interface. Traffic channels come into use once a call has been set up using the control channels. Voice and user data channels typically use dedicated resources, i.e., the channel can be used only by a single mobile device, whereas control channels use shared resources, i.e., the channel can be accessed by multiple users. Voice channels generally do not carry identifying information about the wireless telephone or transmitter in the transmission. For some wireless location applications this distinction can make the use of control channels more cost effective than the use of voice channels, although for some applications location on the voice channel can be preferable.
The following paragraphs discuss some of the differences in the air interface protocols:
AMPS—This is the original air interface protocol used for cellular communications in the U.S. and described in TIA/EIA Standard IS 553A. The AMPS system assigns separate dedicated channels for use by control channels (RCC), which are defined according to frequency and bandwidth and are used for transmission from the BTS to the mobile phone A reverse voice channel (RVC), used for transmission from the mobile phone to the BTS, may occupy any channel that is not assigned to a control channel.
N-AMPS—This air interface is an expansion of the AMPS air interface protocol, and is defined in EIA/TIA standard IS-88. It uses substantially the same control channels as are used in AMPS but different voice channels with different bandwidth and modulation schemes.
TDMA—This interface, also known as D-AMPS and defined in EIA/TIA standard IS-136, is characterized by the use of both frequency and time separation. Digital Control Channels (DCCH) are transmitted in bursts in assigned timeslots that may occur anywhere in the frequency band. Digital Traffic Channels (DTC) may occupy the same frequency assignments as DCCH channels but not the same timeslot assignment in a given frequency assignment. In the cellular band, a carrier may use both the AMPS and TDMA protocols, as long as the frequency assignments for each protocol are kept separated.
CDMA—This air interface, defined by EIA/TIA standard IS-95A, is characterized by the use of both frequency and code separation. Because adjacent cell sites may use the same frequency sets, CDMA must operate under very careful power control, producing a situation known to those skilled in the art as the near-far problem, makes it difficult for most methods of wireless location to achieve an accurate location (but see U.S. Pat. No. 6,047,192, Apr. 4, 2000, Robust, Efficient, Localization System, for a solution to this problem). Control channels (known in CDMA as Access Channels) and Traffic Channels may share the same frequency band but are separated by code.
GSM—This air interface, defined by the international standard Global System for Mobile Communications, is characterized by the use of both frequency and time separation. GSM distinguishes between physical channels (the timeslot) and logical channels (the information carried by the physical channels). Several recurring timeslots on a carrier constitute a physical channel, which are used by different logical channels to transfer information—both user data and signaling.
Control channels (CCH), which include broadcast control channels (BCCH), Common Control Channels (CCCH), and Dedicated Control Channels (DCCH), are transmitted in bursts in assigned timeslots for use by CCH. CCH may be assigned anywhere in the frequency band. Traffic Channels (TCH) and CCH may occupy the same frequency assignments but not the same timeslot assignment in a given frequency assignment. CCH and TCH use the same modulation scheme, known as GMSK. The GSM General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE) systems reuse the GSM channel structure, but can use multiple modulation schemes and data compression to provide higher data throughput. GSM, GPRS, and EDGE radio protocols are subsumed by the category known as GERAN or GSM Edge Radio Access Network.
UMTS—Properly known as UTRAN (UMTS Terrestrial Radio Access Network), is an air interface defined by the international standard third Generation Partnership program as a successor to the GERAN protocols. UMTS is also sometimes known as WCDMA (or W-CDMA), which stands for Wideband Code Division Multiple Access. WCDMA is direct spread technology, which means that it will spread its transmissions over a wide, 5 MHz carrier.
The WCDMA FDD (Frequency Division Duplexed) UMTS air interface (the U-interface) separates physical channels by both frequency and code. The WCDMA TDD (Time Division Duplexed) UMTS air interface separates physical channels by the use of frequency, time, and code.
All variants of the UMTS radio interface contain logical channels that are mapped to transport channels, which are again mapped to W-CDMA FDD or TDD physical channels. Because adjacent cell sites may use the same frequency sets, WCDMA also uses very careful power control to counter the near-far problem common to all CDMA systems.
Control channels in UMTS are known as Access Channels whereas data or voice channels are known as Traffic Channels. Access and Traffic Channels may share the same frequency band and modulation scheme but are separated by code. Within this specification, a general reference to control and access channels, or voice and data channels, shall refer to all types of control or voice and data channels, whatever the preferred terminology for a particular air interface. Moreover, given the many types of air interfaces (e.g., IS-95 CDMA, CDMA 2000, UMTS, and W-CDMA) used throughout the world, this specification does not exclude any air interface from the inventive concepts described herein. Those skilled in the art will recognize other interfaces used elsewhere are derivatives of or similar in class to those described above.
GSM networks present a number of potential problems to existing Wireless Location Systems. First, wireless devices connected to a GSM/GPRS/UMTS network rarely transmit when the traffic channels are in use. The use of encryption on the traffic channel and the use of temporary nicknames (Temporary Mobile Station Identifiers (TMSI)) for security render radio network monitors of limited usefulness for triggering or tasking wireless location systems. Wireless devices connected to such a GSM/GPRS/UMTS radio network merely periodically “listen” for a transmission to the wireless device and do not transmit signals to regional receivers except during call setup, voice/data operation, and call breakdown. This reduces the probability of detecting a wireless device connected to a GSM network. It may be possible to overcome this limitation by actively “pinging” all wireless devices in a region. However, this method places large stresses on the capacity of the wireless network. In addition, active pinging of wireless devices may alert mobile device users to the use of the location system, which can reduce the effectiveness or increase the annoyance of a polling location-based application.